松本 洋介

A characteristic heavy-ion signature observed in the vicinity of the Martian ionosphere during passages of Corotating Interaction Region (CIR) structures in solar wind is reported. We analyzed data obtained by the IMA/ASPERA-3 onboard the Mars Express (MEX) from September to October 2007. We compared the solar wind velocity at Mars derived from a shifted Maxwellian fitting to the IMA data with time-shifted Advanced Composition Explorer satellite data taken at ∼1 AU to the Martian orbit. Using the derived solar wind velocity, we identified four CIR structures passing through Mars quasiperiodically. Coinciding with the CIR passages, the IMA observed heavy-ion flux enhancement in the vicinity of the Martian ionosphere. The heavy-ion energies reach 100 eV and sometimes up to approximately several kiloelectron volts. Observed ion velocity distribution functions show that they are mainly precipitating toward the Martian ionosphere. The flux of the precipitating ions is typically 105-106 (104-105) cm-2 s-1 for the energy range of 50-500 eV (≥500 eV) and it becomes by one order of magnitude higher in one event. While the flux level is consistent with a previous model prediction of sputtering ions, the intermittent occurrence of the heavy-ion precipitation differs from conventional expectation of constant precipitation. These results suggest that the efficiency of the sputtering process in the Martian atmospheric escape is highly variable with dynamic solar wind variations. Copyright 2011 by the American Geophysical Union.

We present a new numerical scheme for solving the advection equation and its application to the Vlasov simulation. The scheme treats not only point values of a profile but also its zeroth to second order piecewise moments as dependent variables, and advances them on the basis of their governing equations. We have developed one- and two-dimensional schemes and show that they provide quite accurate solutions compared to other existing schemes with the same memory usage. The two-dimensional scheme can solve the solid body rotation problem of a gaussian profile with little numerical diffusion. This is a very important property for Vlasov simulations of magnetized plasma. The application of the scheme to the electromagnetic Vlasov simulation of collisionless shock waves is presented as a benchmark test.

Recently, numerical techniques for Vlasov simulations are advanced, and they are applied to cross-scale coupling in space plasmas. This paper presents recent attempts to understand particle kinetics in macroscopic phenomena such as magnetic reconnection and the Kelvin-Helmholtz instability via Vlasov codes, suggesting that the Vlasov simulation would be a powerful approach for studies of cross-scale coupling in space plasmas.